Hand built steel bike frame
Hand built steel bike frame
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Project Overview
Project Overview
During my time at Cal Poly, I was a member of the bike framebuilders club and created an all purpose steel frame. My goal was to build a bike that could be used for mountain biking, gravel biking, commuting, bike packing, and more. The frame was built from straight gauge chromoly tubing with a mix of parts from Paragon, Framebuilder Supply, and 3D printed stainless steel yoke of my own design. Some other key design features a steep unbent seat tube and clearance for 2.4" tires and a 40t chainring.
Design Specifications
Design Specifications
For the design of the frame, versatility was the most important aspect. I wanted a bike that could be used on various terrain and surface types, from road to gravel to moderate singletrack (basically anything my Stumpjumper Evo wasn't suited for). To create an "all-purpose" bike, the frame would need relatively neutral geometry and high component compatibility. I wanted geometry that would give me an upright riding position, efficient pedaling, and a natural steering feel. My component compatibility goals included space for 29x2.4" tires, a 40-tooth chainring, and a 240mm dropper post. This would allow the bike to take on many configurations, ranging between mountain bike, gravel bike, and commuter bike setups. The aesthetics of the frame were also considered but were lower on my list of priorities.
Design Process
Design Process
I designed the frame using a combination of 3D modeling in SolidWorks and 2D drawings in a bike-specific design program called BikeCAD. The simple front triangle could be dimensioned in 2D, whereas a 3D model of the rear triangle was required due to its complexity. The overall geometry was lightly based on existing bikes, such as the Salsa Fargo and Surly Karate Monkey, but had several changes to fit my long legs and short torso. I shortened the chainstays and front triangle, steepened the seat tube, and increased the stack height, which would give me the upright riding position I wanted. With the component compatibility goals above, namely a 2.4" tire with up to a 40-tooth chainring, I determined I would need a yoke, which helps the chainstay navigate the tight space between the tire and chainring. After seeing a sleek application of a 3D-printed titanium yoke at the Sea Otter Classic bicycle expo, I wanted to try something similar with steel. After a few different design iterations to get the geometry and aesthetic I wanted, I arrived at a final yoke design. The design features smooth sweeping curvature and tabs to mate with the chainstay tubing, which are convenient when fixturing and welding the frame. Without being able to test the 3D printed part before using it, I was wary of its strength compared to a conventionally manufactured version. To be safe, I set a high wall thickness of .07", double that of the tubing.
2D design in BikeCAD
Yoke Design in SolidWorks
Rear Triangle Assembly
Manufacturing
Manufacturing
The first part manufactured was the yoke. I sent off my design to a 3D printing service with a two-week turnaround including shipping, for less than $40. With this price and the increased design freedom, using a 3D-printed yoke was an easy decision. The part had only one minor defect on the cross piece, possibly due to powder slumping during printing.
Other parts including headtube, dropouts, and bottom bracket were sourced from Paragon Machine Works and Framebuilder Supply. All tubing is straight gauge 4130 (chromoly) steel with a wall thickness of .035" and was purchased from Wicks Aircraft.
The next step in manufacturing was to prepare the tubing for welding. I bent the tubing using a Cobra Framebuilding TOOB bender that the bike builders club has available for members. Then tubes were cut to length and mitered (also called coped or notched) using a hole saw in a manual milling machine.
The last steps involved fixturing the frame parts in an Anvil frame jig and tig welding all the joints. For the stainless yoke, I back-purged the inside of the tubes with argon to prevent oxidation.
3D printed stainless steel yoke
Double bend chainstays
Mitering tubing on Bridgeport mill
Drilled and slotted seat tube
Seat stay miter finished by hand filing
Front triangle tubing fixtured in Anvil frame jig
Yoke and chainstays fixtured in place
Seat and down tubes mitered to fit bottom bracket
Welding...
Front triangle done
Welding...
More Welding...
Welding done!
Brazing on cable guides, bottle bosses, rack mounts, and dropper post port.
First assembly
First singletrack ride!
Rides smooth (for a rigid mtb at least!)
Glamour Shots
Glamour Shots
Post-Build Updates and Testing
Post-Build Updates and Testing
For now, I left the frame unpainted but will be getting it powder-coated in the future. I'm currently enjoying the rusty patina look, but eventually will want to prevent long-term rusting.
Additionally, I am still experimenting with component compatibility and optimal spec. For example, the Ritchey fork in the above photos has a 435mm axle-to-crown measurement, which is too short for the geometry I wanted, since it affects headtube angle, bb height, etc. I designed the frame to work with a Surly Karate Monkey fork (483mm A to C), which I did not realize had been discontinued. After the above photos were taken, I was able to dig one up in the back stock at Onion River Outdoors where I worked at the time, and will be testing it soon.
Lastly, I have a OneUp 240mm dropper post to install, but a quick test fit ensured it fits perfectly.
Results
Results
The Karate Monkey fork proved to refine the handling characteristics. Most notably, the steering felt more predictable and direct due to the changes in headtube angle and mechanical trail. Additionally, with the stack height increased to my design goal of 696, my body position and weight distribution on the bike were improved. The new fork also increased the bottom bracket height, reducing the chance of pedal strikes.
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